as a part of my research project, I needed a chapter from the book Lowland Maya Settlement Patterns. The copies at both ASU and U of A were checked out, so I headed to Amazon.com.
Delightfully, there were seven or eight used copies available, ranging in price from $10 to $54. I snapped up the $10 copy, and it arrived just four days later. I wouldn't do this for every source I might need, but this looks like a book I would own, so I went for it.
In Amazon's Marketplace (books sold by Amazon customers), one can find numerous out-of-print archaeology and anthropology books and reports, often for very reasonable prices. There are a surprising number of excavation reports available (e.g. Archaeological investigations at the Arroyo Hondo site: Third field report, 1972).
You can even browse Anthropology or Archaeology books by subtopic. Take a look at these starting points, and use the subtopics in the left-hand nav:
Archaeology Books at Amazon
Anthropology Books at Amazon
You can sort the results by price, customer rating, etc. I've also found several of my textbooks over the last few years on Amazon, usually paying significantly less than the used prices at the university bookstore.